Effect of Carbon and Nitrogen Availability on the N Uptake Pathway of Soil Microorganisms.

Microorganisms can take up nitrogen (N) in different forms, namely as ammonium (NH₄), nitrate (NO₃) or small organic molecules, such as amino acids. NH₄ is considered the dominant form taken up by soil microorganisms. Current models of N mineralization implicitly assume that all the N in organic substrates is mineralized to NH₄ before assimilation by microorganisms. However, there is evidence that organic molecules may contribute considerably to the N nutrition of soil microorganisms. In this direct route, large polymers are hydrolyzed by extracellular enzymes into smaller units, such as amino acids, which are then taken up directly without being deaminated first. Which route is operative becomes important in the interpretation of gross N mineralization data from organic residue.

In an ongoing project we are testing the hypothesis that the N uptake pathway is dependent on the form of N present and the availability of carbon (C) and N. In a first step we are developing assays to measure the membrane bound enzymes responsible for the uptake of NH₄ by soil microorganisms. So far, these assays have been used only in studies with pure cultures. In a second step we are combining these assays with the measurement of extracellular enzyme activity and pool dilution methods to determine gross N mineralization and amino acid uptake.